Plastic Injection Molding With Moveable Insert Members

ABSTRACT

A method and system for producing hollow rib structures for trim components and panels using gas assisted injection molding. Movable insert members are provided in the mold cavity, particularly at the ends of the structural rib members. After the plastic material is injected into the mold cavity, the plastic is packed in the mold, and the insert members are locked in position. Selectively activatable locking mechanisms are used to lock up the insert members. Thereafter, gas or another fluid is introduced into the rib members in order to provide hollow channels therein. Movement of the insert members provides a recess or groove for placement of the displaced resin from the rib members. The displaced resin material completes the formation of the molded plastic article.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This is a continuation-in-part of U.S. patent application Ser.No. 10/249,237 filed on Mar. 25, 2003.

BACKGROUND OF INVENTION

[0002] The present invention relates to plastic injection molding andmore particularly to plastic injection molding using gas assist and theformation of hollow rib members on plastic injection molded parts.

[0003] In the plastic injection molding art, the usual challenges facingthe product designer include designing an article having the requisitestrength for the product application and satisfactory surface finish, aswell as avoiding excessive weight, surface distortions, and increasedcycle time. For flat or thin products, it is typical to include one ormore rib members in the design to provide relative strength andstructure for the molded article. The rib members are typically thickerthan the molded article which increases the weight, material usage, andcycle time of the article, and often induces sink marks and othersurface defects due to a thermal gradients in the area of the thickenedsection.

[0004] It is known in the plastic molding art to use pressurized gas,such as nitrogen, in conjunction with plastic injection molding ofarticles. Pressurized gas serves several purposes. The gas allows thearticle or rib structure to have hollow interior portions which resultin savings in weight and material, thereby reducing costs. Thepressurized gas also applies an outward pressure to force the plasticagainst the mold surfaces while the article solidifies. This helpsprovide a better surface on the molded article and also reduces oreliminates sink marks and other surface defects. The use of pressurizedgas also reduces the cycle time as the gas is introduced and/or migratesto the most fluent inner volume of the plastic and replaces the plasticin those areas which would otherwise require an extended cooling cycle.The pressure of the gas pushing the plastic against the mold surfacesfurther increases the cooling effect of the mold on the part, thussolidifying the part in a faster manner and reducing the overall cycletime.

[0005] Where the rib members or other portions of the article in whichthe gas is being introduced are elongated, it is often difficult toprovide a satisfactory molded article. For example, if the pressure ofthe gas is too great as it enters the mold cavity, there is a risk thatit may rupture or blow out the plastic within the mold cavity, i.e. thegas is not contained within the plastic. Also, it is often difficult tohave the gas migrate along the full length of an elongated, thickerplastic section, thus creating a product which has an uneven thicknessand cooling cycle.

[0006] One manner which has been developed in order to overcome some ofthe above-mentioned problems is shown in U.S. Pat. No. 5,098,637. Inthat process, a secondary cavity (a/k/a “spillover” cavity) is providedat one end of the molded part or elongated rib member in order tocollect and contain the more fluent plastic material which is forced outof the article or rib member by the pressurized gas. Another system isshown in U.S. Pat. No. 5,885,518.

SUMMARY OF INVENTION

[0007] An object of the present invention is to provide an improvedmethod of gas assisted injection molding. It is another object of thepresent invention to provide an improved method and system for injectionmolding plastic parts with structural rib members.

[0008] It is a still further object of the present invention to providea method and system for gas assist injection molding which eliminatesthe need for spillover cavities and the subsequent capture andregrinding of excess plastic material from a mold cavity. It is anadditional object of the present invention to provide a moveable insertin a mold cavity and allow packing of the plastic material in the moldfor better dimensional and surface effects.

[0009] In accordance with the objects of the present invention, one ormore movable insert members are provided in the mold cavity. The insertmembers are particularly located in portions of elongated rib members inwhich pressurized gas is to be introduced in order to provide hollowportions in them. The insert members can be held in place by spring orother biasing members or mechanisms which are adapted to be overcome bythe force of the plastic and pressurized gas when the gas is introducedinto the rib members.

[0010] A full-shot of plastic material is first injected into the moldcavity. The plastic material is then packed in the mold by additionalpressure plastic injection from the molding machine. Thereafter,pressurized gas is introduced into at least one of the rib members. Asthe gas channel is formed in the rib member(s) and the fluent plasticmaterial in the center of the rib member(s) is pushed along the ribmember(s) and out the opposite end(s), the spring or biasing force onthe insert member(s) in one embodiment of the invention is overcome. Asthe insert member(s) retracts, the plastic material fills the entirearticle defining mold cavity. The displaced plastic material flows intothe area formerly occupied by the insert member(s), creating anadditional rib or part of the molded article.

[0011] Once the entire article is formed, the plastic is allowed to cooland solidify. The injected gas pressure is then relieved (vented orexhausted) from the formed article and the article is ejected or removedfrom the mold.

[0012] In one preferred embodiment, the insert member is held in itsinitial position by spring tension, pressurized cylinders, dampingmechanisms, or the like. These tensioning or biasing mechanisms providea sufficient biasing force to resist the level of force caused bypressure in the mold from the initial injection of the plastic material.This maintains the nominal wall thickness of the article being moldedthroughout the full extent of the mold with the exception of the thickersection of the rib members. Once the cavity is full, pressurized gas isintroduced at the end of the rib members opposite the insert members.The force of the gas pressure and the resin forced from the rib membersovercomes the biasing force of the insert members.

[0013] In order to withstand the additional packing pressure applied tothe plastic material in the mold, the insert members are fixed or lockedinto position by use of a releasable locking mechanism. The lockingmechanism can be a pneumatic, hydraulic, or electrical mechanism, or canutilize a controllable fluid which can be solidified by application ofan electric current. The insert member can then be released for movementwhen the gas is injected by the use of various devices, such as positionsensors, cavity pressure sensors, timers, or the like, or thedeactivation of the electric current. The release or deactivation of thelocking mechanism can be based or triggered in a number of differentways, such as by attainment of certain pressures in the mold, processtiming sequences, linear movement of the injection screw, etc.

[0014] The above objects and other objects, features, and advantages ofthe present invention are readily apparent from the following detaileddescription of the preferred embodiments for carrying out the inventionwhen taken in accordance with accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

[0015]FIG. 1 is a schematic diagram of a typical plastic injectionmolding system.

[0016]FIG. 2 is a schematic diagram of an article being molded in aconventional gas assisted plastic injection molded system.

[0017]FIG. 3 is a perspective view of a molded part or article accordingto the present invention.

[0018]FIG. 4 is a cross-section of a structural rib member on a partmade in accordance with the present invention, the cross-section beingtaken along lines 4-4 in FIG. 3 and in the direction of the arrows.

[0019]FIG. 5 is a cross-sectional view of a mold utilizing an embodimentof the present invention.

[0020]FIG. 6 is a partial cross-sectional view showing an insert memberin accordance with the present invention.

[0021]FIG. 7 is another view of an insert member in accordance with thepresent invention, the view being taken in the direction of the arrows7-7 in FIG. 6.

[0022]FIG. 8 is another view of the insert member and mechanism as shownin FIG. 7, with the insert member being displaced in response to theintroduction of gas in a rib member.

[0023]FIG. 9 is a flow diagram indicating the basic steps involved inthe practice of the method of the present invention.

[0024]FIG. 10 is a schematic diagram depicting a locking mechanism foran insert member.

[0025]FIG. 11 illustrates an alternate mechanism and system for lockingan insert member in place.

[0026]FIG. 12 is a flow diagram of an alternate embodiment of theinvention.

[0027]FIGS. 13-14 illustrate a still further embodiment of theinvention.

DETAILED DESCRIPTION

[0028] The present invention is particularly suitable for producinghollow rib structures for structural reinforcement of decorative ornon-decorative trim components and panels using gas-assist plasticinjection molding.

[0029] As shown in FIG. 1, a conventional plastic injection moldingsystem 20 is shown. The system 20 includes an injection molding machine22, a mold 24, and a source of gas 26 for the gas assisted injectionmolding. As is well known in the injection molding field, injectionmolding machine 22 has a hopper 28 in which plastic pellets are added tothe machine. The injection molding machine includes a mechanism formelting the plastic material and a screw-type ram (not shown) in thebody of the machine. The molten plastic material is injected through anozzle 30 into the mold 24.

[0030] Mold 24 includes two mold halves 24A and 24B. The two mold halvesare connected to platens 32 and 34 which separate and come together asone or more is moved on the injection molding machine.

[0031] The gas used in injection molding is typically an inert gas, suchas nitrogen, and is supplied from a storage canister or system (notshown) to a gas injection controller 26. The controller 26 regulates theflow of gas through a conduit 36 or the like into the mold 24.

[0032] It is to be understood that the present invention can be usedwith any plastic injection molding machine 22, any gas assist controller26, and virtually any type of mold 24. In this regard, a schematicdrawing of a representative mold 24 is shown in FIG. 2.

[0033] An article forming cavity 40 is provided between the two moldhalves 24A and 24B. The plastic material 42 is injected through themachine nozzle 30, and through a bushing 44 into the mold cavity 40.After all or a substantial portion of the plastic material 42 isinjected into the mold cavity 40, gas from the controller 26 isintroduced into the mold cavity. The gas forces the plastic materialinto all portions and into all surfaces of the mold cavity 40 andprovides a hollow interior 46.

[0034] Once the plastic material 42 has cooled and solidified in themold, the gas is vented, and the mold is then opened in order to allowthe part to be removed. In this regard, there are many mechanism systemsknown for relieving the gas pressure in the mold, i.e. venting orexhausting the gas from the mold, and any conventional system and methodcan be utilized. For example, gas can be vented back through the conduit36 to the controller, or the nozzle 30 can be backed off from thebushing 44 creating a “sprue break.” Also, plastic formed articles aretypically assisted in being removed from the mold by one or more ejectorpins (not shown).

[0035] As indicated, the present invention is particularly suitable forproducing plastic injection molded trim components and panels having ribmembers. A representative article of this type is shown in FIG. 3 andidentified generally by the reference numeral 50. As shown, the article50 has a relatively thin thickness 52 relative to its width and lengthand has a plurality of rib members 54 on one surface (typically the backor non-visible surface). The outer surface 50A of the article 50 is thesurface which is exposed to view when the part is used or installed inits appropriate environment. The article 50 could be, for example, acomputer housing, a panel for a door or other article of a vehicle, orthe like. Rib members 54 are provided on the opposite side 50B and arehidden from view during normal use of the article 50. Although four ribmembers 54 are shown on the article 50, any number of rib members may beutilized. The rib members in particular provide structural support forthe panel-type article 50.

[0036] Since the rib members 54 are typically thicker in dimension thanthe width or thickness of the article 50, gas assist injection moldingtechniques and procedures are typically utilized in order to hollow outthe rib members. This reduces the thickness of the rib members, therebysaving material cost, reducing cycle times, and minimizing oreliminating surface defects, such as sink marks, on the viewed surface50A.

[0037] The term “gas” used herein means any gas body gaseous duringnormal temperature and pressure, such as nitrogen, carbon dioxide air.Preferably, the gas is an inert material, such as nitrogen or carbondioxide. The gas can be provided from any conventional source, such as astorage tank, bottled gas, or the like. It is also possible inaccordance with the present invention to utilize another fluid, such aswater, in place of the gas. These other fluid systems are known in theart and do not need to be discussed in more detail here.

[0038] A rib member 54 is shown in cross-section in FIG. 4. The article50 has a certain thickness 52 which requires a certain cooling and cycletime. In order to provide requisite structural support, the rib member54 typically has thicker portions, such as 54A. In order to lighten theof the rib member 54 and reduce its thickness, a channel 56 is providedin the rib member by the introduction of the pressurized gas or otherfluid.

[0039] Although the injection molding system is shown utilizing a singlebushing member 44 positioned between the nozzle 30 of the injectionmolding machine 22 and the mold 24, it is understood that anyconventional injection molding system could be utilized in transportingthe molten plastic material from the nozzle 30 to the mold cavity 40.For example, a hot runner system with one or more manifolds and aplurality of bushings could be utilized. Other systems and mechanismsknown in the art could also be utilized.

[0040] A representative molding system in which the present invention isutilized is shown in FIG. 5. Further details of this embodiment areshown in FIGS. 6-8. The basic steps in this embodiment of the inventionare set forth in FIG. 9.

[0041] The present invention can be used with all types of plasticmaterials, particularly thermoplastic materials, which generally may beinjection molded. These materials can be, for example, low or highdensity polyethylene, polyurethane, polypropylene, polystyrene,acrylonitrile butadiene styrene (ABS) resin, SAN resin, polyvinylchloride, polymethyl methacrylate, and the like. Materials such aspolycarbonate, polyester, acetyl, polyacetyl and nylon also may be used,as well as ethylene copolymers such as ethylene-vinyl acetate copolymer,ethylene-ethyl acrylate copolymer, and the like. Furthermore, any ofthese resins may contain fillers, such as glass fiber, powdered glass,calcium carbonate, calcium sulfate, talc, mica, and the like.

[0042] In accordance with the present invention, molten plastic materialis injected from a machine nozzle 30 to a bushing 44 and into a moldcavity 60 in a mold 62. The two mold halves 62A and 62B are adapted toseparate along part line 64 in order to allow the formed plastic articleto be removed. A gas regulated by gas controller 26 is introducedthrough conduits 36 into ends 66A of rib members 66 on the plasticarticle 68, which is formed in the mold cavity 60. Movable ordisplaceable insert members 70 are provided in the mold cavity 60, andare positioned such that they are situated at the opposite ends 66B ofthe rib members 66. Pressurized gas is introduced into the rib members66 and forms channels 72 essentially through the centers of thecross-sections and throughout their lengths.

[0043] Prior to injection of the gas, the plastic material in the moldis packed the introduction of more plastic material, and at a higherpressure. This insures excellent surface quality for the molded articleor product. Also, once the plastic packing is completed, a shut offvalve (not shown) in the nozzle closes off the plastic supply prior tothe injection of the gas. This prevents plastic material from beingforced or pushed back into the barrel of the injection molding machine.

[0044] The gas is injected into the resin in the mold at a pressurehigher than the plastic injection pressure and typically at a muchhigher pressure. For example, the initial plastic injection fillpressure could be 100-200 psi, which will normally increase to 500-1000psi or higher with packing pressure. The gas injection could be1000-1500 psi (if, for example, the plastic material was polypropylene)and 3000-7000 psi (if, for example, engineering grade resins were used).In general, the pressure of the gas should be sufficient to provide acommercially acceptable surface and finish on the plastic part. When theplastic is packed in the mold cavity, the pressure is raised severaltimes over the initial injection pressure and additional plasticmaterial is packed into the mold cavity. The pressure of the gas istypically higher than the plastic packing pressure.

[0045] The insert members 70 are held in position in the mold cavity bya biasing or tensioning mechanism 80. In this regard, as shown in FIGS.7 and 8, the insert member 70 is typically elongated in shape and thus anumber of biasing mechanisms 80 are provided along its length. A biasingforce is provided by the biasing mechanism 80 such as by one or morecoil spring members 82 as shown in FIGS. 6-8. The spring members 82provide a biasing force against the insert member 70 sufficient tooppose the force of the plastic material 68 when it is injected into themold cavity 60. The biasing force or spring tension holding the insertmember in place preferably should be sufficient to resist a force of150-200 psi (10.4-13.8 bars).

[0046] It is to be understood that any type of biasing, tension, orforce-type member or mechanism can be used to hold the insert members inplace. For example, gas or air charge cylinders can be utilized in placeof the coil spring members 82, or a conventional dampening mechanismcould be utilized. The biasing member has to provide a sufficient forceor load on the insert member to resist the level of force caused bypressure in the mold cavity during the injection of the plasticmaterial. This maintains the nominal wall thickness of the article 68 tothe end of the flow length with the exception of the thicker section ofthe rib member 66.

[0047] As shown in FIGS. 7 and 8, the introduction of pressurized gasthrough the rib member 66 forms channel 72 in the rib member and forcesplastic material against the insert member forcing it to move downwardlyin the direction of the arrow 84 in FIG. 8. The additional space formedin the mold cavity by the movement of the insert member is filled withmolten plastic forced from the rib member. The additional plastic formsan additional rib member 90 adjacent the end of the now hollow ribmember 66. Additional rib members of this type are shown in FIG. 3 andidentified by the reference numeral 90′. The additional rib members 90′are positioned at the end of the rib members 54 and can provideadditional support and structural stability to the article 50. Dependingon the amount of displaced plastic material and the gas injectionpressure, it is also possible to provide a gas channel and thus a hollowstructure through a portion of the new rib member 90.

[0048] Although the invention is primarily directed to the formation ofstructural rib members on plastic injection molded articles, such astrim components and panels, it is also possible to use the presentinvention to produce or provide portions of articles necessary to allowappropriate molding. For example, it is often necessary to providemolded portions of an article in order to prevent undercuts and the likeand thus to assist in the production of molds which can be opened andclosed more easily and allow the parts to be removed and ejected.

[0049] The thickness of the additional rib member 90 preferably shouldnot be greater than the maximum thickness of any other part of themolded article (for minimum cycle times). Also, depending on thestrength of the rib members 90 needed for the particular application andmolded part, the rib members can be increased in width. In order toinsure that size and thickness of the added portion or rib member 90remain within predetermined limits, a stop member (not shown) can beutilized to limit the travel of the insert member 70.

[0050] Although the portion formed on the molded article by thedisplacement of the insert member is referred to herein as a rib member,it is to be understood that the portion can have the shape and size toadd virtually any structure to the article, such as, for example, aboss, a snap fit member, etc. The portion can also be used to addadditional strength to the article.

[0051] The size, shape and volume of the insert member and resultantadded portion formed on the article should be predetermined, so that allejected plastic material from the rib member can be accommodated.Similarly, the size of the added portion should be sufficient toaccommodate for volumetric differences in the plastic material from theshot-to-shot (i.e. from cycle to cycle). The biasing mechanism 80 allowsfor such volumetric differences.

[0052] As an alternate embodiment, it is also possible to introduce thegas into the rib member 66 at a location inbetween the ends 66A and 66B.In such an embodiment, it is also possible to position insert membersand form additional article portions at both ends of a rib member.

[0053] The basic steps in the process in accordance with the presentinvention are set forth in the flow diagram in FIG. 9. The basic methodis referred to generally by reference numeral 100. As a first step, theplastic material is injected into the mold cavity in the mold. This isreferred to by the reference numeral 110 in FIG. 9. This first step iscompleted after the precise mold cavity size and shape is selected, themold cavity is machined in the mold, the appropriate hot runner system,bushing, and manifold mechanism is selected and provided, the requisitesize of the injection molding machine is selected, the type of plasticor resin material for the article or part is selected and provided, andthe appropriate processing parameters, such as time, pressures,temperatures, and cycle time, are determined. All of these matters arewithin the ordinary skill in the art.

[0054] The molten plastic resin material is injected into the moldcavity by the injection molding machine until the article cavity is fullof plastic material. In this manner, a “full shot” of plastic materialis injected into the mold cavity. In order to provide better aestheticson the final and insure that all of the dimensions and surface detailsare obtained, the injected plastic material is then packed in the mold.The pressure of the plastic injection is increased and additionalplastic material is forced into the mold cavity. This pressure can beheld for a certain length of time, such as 5-15 seconds, in order toform a solidified outer shell on the product. This is shown in Box 112in FIG. 9.

[0055] In this regard, in order to prevent the insert member from beingdislodged or moving during the additional pressure or packing procedure,the insert members are held or locked into position by the use of anexternal locking mechanism 92 which can be a wedge, pin, or the like 94(see FIGS. 5-6 and 10). The wedge 94 is situated in the mold to be movedinto position preventing movement of the insert member 70. The wedge 94is activated by a activation system 96, such as a pneumatic, hydraulic,electric, or mechanical mechanism or system and set and locked intoposition or released by the use of various devices, such as positionsensors, cavity pressure sensors, timers, and the like (not shown).

[0056] After the mold cavity is full of packed molten plastic resinmaterial, the gas is injected into one or more of the rib members. Thisis shown by Box 120 in FIG. 9. The gas is preferably injected at one endof the rib members opposite the insert members and at a predeterminedtime and pressure. The pressure of the gas is typically higher than thepressure used to pack the plastic in the mold.

[0057] In order to prevent any of the plastic material from being forcedback into the nozzle or injection molding machine during gas injection,a shut-off valve should be provided at, or adjacent to, the nozzle andclosed upon the completion of the packing procedure.

[0058] During the gas injection procedure, the molten plastic materialis displaced from the core or interior of the rib members creating anincrease in pressure on the insert member or members. This pressureovercomes the biasing force caused by the spring mechanisms or the like,forcing the insert members away from their rest positions and retractingto increase the available volume of the cavity. This is shown in Box 130in FIG. 9. The displaced plastic material from the rib members will flowinto the voids formed by the displacement of the insert members. Therange of movement of the insert member is preferably predetermined inorder to provide an additional part or portion on the article of acertain size, shape, and length. At the end of movement of the insertmember, the pressures equalize on the insert member, allowing theexternal gas pressure to provide packing pressure to the molded partthroughout the rib members.

[0059] If desired, once the insert members are displaced and the voidsare filled with the plastic material, an additional packing procedurecan be provided by the introduction of a higher pressurized gas. This isshown as an alternative in Box 132 in FIG. 9.

[0060] Once the mold cavity is filled with plastic material and the gasassist process is completed, the plastic article is allowed to cool andsolidify in the mold. This is shown in Box 140 in FIG. 9. Thereafter,the gas is vented or exhausted from the rib members as shown in Box 150.The venting of the gas from the rib members can be accomplished in anystandard manner known to persons of ordinary skill in the art. Forexample, one method allows the gas to vent back through the end of therib members where the gas is initially introduced. The gas pressure canalso be relieved in a stepped manner over time, or simply allowed todecay.

[0061] For the final step, the mold is opened and the formed plasticpart or article is removed (or ejected) from the mold cavity. This isshown in Box 160 in FIG. 9. Thereafter, the mold is closed and theprocess is repeated.

[0062] In an alternate process, all or a portion of the gas can beinjected into the plastic material after the initial plastic injection(Step 110) and prior to the plastic packing procedure (Step 112). Thiswould allow more of the gas to be introduced into the plastic materialin the mold cavity. The shut off valve in the nozzle also could beactuated prior to the gas injection step in order to prevent plasticfrom being forced back into the injection molding machine. The valvethen could be reopened in order to pack additional plastic material intothe mold cavity. Thereafter, the shut-off valve could be closed again ifthe gas injection is restarted.

[0063] Movement of the insert members may provide a sufficient pressuredrop to obviate the need to shut-off the plastic flow again. Also, aprocess procedure of backing off the clamping pressure by, for example,opening the mold a few thousands of an inch for a few milliseconds couldbe utilized. This would allow the plastic material to flow into the voidcaused by displacement of the insert members and form hollow portionsfilled with gas in the ribs.

[0064] A flow diagram schematically illustrating this alternate processis set forth in FIG. 12. The process is designated generally by thereference numeral 200. At Step 210, the plastic material is injectedinto the mold cavity. Then, in Step 220, all or a portion of the gas isinjected into the plastic material in the mold cavity. Thereafter, inStep 230, the plastic material in the mold is packed by the addition ofadditional plastic material at a higher pressure. This insures that theplastic material entirely fills out the mold cavity and is pressedtightly against all of the surfaces of the mold cavity.

[0065] Then, if all of the gas had not been injected earlier, theremaining amount of pressurized gas is injected into the plasticmaterial in the mold cavity. This is shown in Step 240. Also, after theplastic packing Step 230, the shut-off valve in the nozzle is closed inorder to prevent plastic material from being pushed back into the barrelof the injection molding machine.

[0066] As an alternative, all of the plastic can be injected into themold cavity before any of the gas is injected. The plastic shouldcompletely fill the mold cavity and can be packed if desired. Once theinjection of the gas has begun (or is completed), the insert member canbe moved.

[0067] The injection of the gas should also be continued when the insertmember is moved in order to maintain the appropriate gas pressure in theplastic material and keep it pressed tightly against the walls of themold cavity.

[0068] The insert member or members are moved or displaced forming voidsin the mold cavity and creating additional volume(s) for the plasticmaterial. This is shown in Step 250. A portion or portions of thestill-molten plastic material is displaced or flows into the voidvolumes, as shown in Step 260. It is also possible at this point to openthe mold as described above. This is shown in alternate Step 265. As theplastic material flows or is forced into the void areas, the injectedgas forms hollow portions in the plastic material, particularly alongthe rib members.

[0069] Once the plastic article is fully formed, it is allowed to cooland solidify (Step 270). Once the gas is vented (Step 280) or the gaspressure is otherwise relieved in some manner, the mold is opened andthe molded plastic article is removed or ejected from the mold cavity(Step 290).

[0070] The rib members in which the gas is to be introduced should bedesigned to prevent the gas from escaping from the rib structure intothe nominal wall thickness of the article. Also, the insert membersshould be dimensioned such that the increase in volume in the moldcavity is proportional to the volume of plastic material which isdisplaced by the injection of gas into the rib members. Persons skilledin the art have sufficient knowledge to accomplish these tasks.

[0071] With the present invention, the mold cavity is increased inavailable volume at a predetermined point in the plastic injectionmolding process in order to allow the molten plastic resin from theinterior of the rib members to be displaced by the introduction of thegas. The present invention does not require a secondary cavity externalto the mold cavity for displacement of the plastic resin material. Theinsert members compensate for the volumetric variation due to resinviscosity variation inherent in injection molding processes.

[0072] Also, with the present invention, volumes of plastic materialfrom a secondary cavity do not have to be recycled or reground. Thiseliminates an additional processing step which results from other knownprocesses.

[0073] An alternate mechanism for locking the insert members in place isshown in FIG. 11 and identified generally by the reference numeral 170.The insert member 70′ is biased by spring members 82′ and connected byrod member 172 to a piston member 174. The piston member 174 ispositioned in a chamber or container 176 which is filled with acontrollable fluid 178. Controllable fluids are materials that respondto an applied electric or magnetic field with a change in rheologicalbehavior. Typically, this change is manifested when the fluids aresheared by the development of a yield stress that is more or lessproportional to the magnitude of the applied field. These materials arecommonly referred to as electrorheological (ER) or magnetorheological(MR) fluids.

[0074] Conventional devices that utilize controllable fluids aretypically classified as having either fixed poles (valve mode) orrelatively moveable poles (direct-shear mode). Valve mode devices caninclude servo-valves, dampers and shock absorbers, while direct-sheermode devices can include clutches, brakes, chucking and locking devices.The response time of the fluid damping in these devices can be on theorder of 60-milliseconds as the field is changed.

[0075] With the embodiment and system shown in FIG. 11, coils 180 areconnected to electric current source 182. When the source 182 isactivated, the current through coils 180 causes the fluid 178 to harden,thereby preventing the piston member 174 from moving. This, in turn,holds or locks the insert member 70′ in place. Activation of theelectric current source 182 can be caused in any manner, such as beingtriggered by a pressure sensor 184 positioned in the mold cavity 186. Acontrol system or mechanism 188, which preferably is a microprocessor,is set or programmed to lock up the insert member when a certainpressure is reached in the mold cavity which would otherwise displacethe insert member.

[0076] The locking of the insert member could also be triggered bymeasurement of the movement of the screw ram in the injection moldingmachine, by certain timing considerations, or any other manner known inthe art. For example, a linear transducer could be utilized whichmeasures the axial movement of the screw ram and be connected to amicroprocessor system.

[0077]FIGS. 13 and 14 depict still another embodiment of the invention.This embodiment 300 uses a movable insert member which is positionedalong a portion of a rib member in the molded part and which does notcreate a void or vacant area in the cavity when the insert member ismoved.

[0078] The system 300 includes a mold 302 which has a first member 302and a second member 304, as well as a moveable insert member 310. Theplastic article 312 is formed in a mold cavity 315 positioned betweenthe two mold members 302 and 304.

[0079] Once the plastic material is injected into the mold cavity 315, agas (or other fluid) is injected into the plastic material through pinmember 320. The gas initially forms hollow cavity 325 in the plasticpart, as shown in FIG. 13.

[0080] The insert member 310 is actuated by one or more hydrauliccylinders 330 and moves from an in-mold position 310A in FIG. 13 to aflush-mold position 310B in FIG. 14. The amount of movement “D” of theinsert member can be on the order of 0.020 “to 0.030. ” Once the plasticmaterial and gas are injected into the mold cavity, the insert member ismoved from the position shown in FIG. 13 to the position shown in FIG.14. This allows the gas to expand in the plastic material and to forcethe plastic material into the increased volume. This also allows the gasto move to the furthest end of the rib and form the requisite gaschannel 325′. This system and process insures that there will not be anyblowout of the gas through the plastic material, and also prevents theformation of sink marks on the visible surface of the molded article.

[0081] The insert member 310 can be moved from position 310A to 310Bsimply by the pressure of the injected gas, or it can be moved byactuation of the hydraulic cylinder. Also, the pressure on the hydrauliccylinder can simply be relieved which would then allow the gas pressureto move the plastic and force the insert member to its second position.

[0082] Although the present invention has been described above withreference to formation of hollow rib structures and trim components orpanels, it is to be understood that the present invention can be used inany gas assist injection molding processes.

[0083] While the invention has been described in connection with one ormore embodiments, it is to be understood that the specific mechanisms,processes and procedures which have been described are merelyillustrative of the principles of the invention, numerous modificationsmay be made to the methods and apparatus described without departingfrom the spirit and scope of the invention as defined by the appendedclaims.

1. A method for making an injection molded plastic article with at leastone hollow portion, said method comprising the steps of: injecting aquantity of a plastic material into a mold cavity, the quantity ofplastic material sufficient to completely fill the mold cavity; packingsaid plastic material in said mold cavity; injecting a fluid materialinto said packed plastic material in said mold cavity; increasing thevolume of said mold cavity; displacing by said fluid material at least aportion of said plastic material into said increased volume and therebycreating a hollow portion in said plastic material forming said article;allowing the completed plastic article to cool and solidify in the moldcavity; exhausting said fluid material from the hollow portion of theplastic article; and removing the plastic material from the mold cavity.2. The method as described in claim 1 wherein said fluid material is agas.
 3. The method as described in claim 1 wherein said plastic articlehas at least one rib member and said hollow portion is in said ribmember.
 4. The method as described in claim 1 wherein said quantity ofplastic material is injected at a first pressure and said plasticmaterial is packed in said mold cavity at a second pressure greater thansaid first pressure.
 5. The method as described in claim 4 wherein saidfluid material is injected into said packed plastic material at a thirdpressure.
 6. The method as described in claim 5 wherein said thirdpressure is greater than said second pressure.
 7. The method asdescribed in claim 1 wherein the step of increasing the volume of saidmold cavity comprises removing an insert member from said mold cavity.8. The method as described in claim 7 wherein said step of removing saidinsert member comprises displacing said insert member by pressure fromsaid displaced plastic material or said injected fluid material.
 9. Themethod as described in claim 8 wherein said insert member is biased in adirection against displacement.
 10. The method as described in claim 8wherein said step of displacing said insert member comprisesdeactivating an electric current.
 11. A method for making an injectionmolded plastic article having a hollow rib member, said methodcomprising the steps of: injecting a full shot of plastic material intoa mold cavity at a first pressure, said mold cavity having a firstportion forming said rib member on the completed plastic article andsaid mold cavity having a displaceable insert member in said firstportion; packing said plastic material in said mold cavity at a secondpressure greater than said first pressure; injecting a gas into theplastic material in said first portion and displacing said displaceableinsert member; and allowing said plastic material to solidify.
 12. Themethod as described in claim 11 further comprising exhausting said gasfrom said plastic material and removing said completed plastic articlefrom said mold cavity.
 13. A method for displacing a displaceable insertmember in a mold cavity filled with plastic material, said insert memberbeing biased to its rest position and held in place at its rest positionby a moveable piston member, and said piston member being positioned inan electrical activable fluid, said method comprising the steps of:deactivating the electrical activable fluid; and displacing said insertmember by injecting a gas into said plastic material to form a hollowrib member.
 14. A system for injection molding a plastic article havinga hollow portion therein, the system comprising: a mold member having amold cavity for defining an article, said cavity having at least oneinsert member positioned therein, and a biasing mechanism to hold saidinsert member in position during injection of plastic material; aninjection molding machine for injecting a quantity of molten plasticmaterial into the mold cavity; a locking mechanism for selectivelypreventing movement of said insert member; a gas introduction mechanismfor introducing pressurized gas into at least a portion of the plasticmaterial in the mold cavity; wherein the introduction of gas into theplastic material displaces said insert member against the force of saidbiasing mechanism and allows formation of the complete molded plasticarticle.
 15. The system as described in claim 14 wherein said portion ofthe plastic material in which gas is introduced comprises at least oneelongated rib member.
 16. The system as described in claim 15 wherein atleast two rib members and at least two insert members are provided, gasis introduced into each of said rib members, and each of said insertmembers are displaced by plastic material.
 17. The system as describedin claim 14 wherein said biasing mechanism comprises a spring mechanism.18. The system as described in claim 14 wherein said locking mechanismcomprises a wedge or pin member.
 19. The system as described in claim 14wherein said locking mechanism comprises a piston member positioned in aelectrifiable fluid, said piston member being in operable associationwith said insert member, and wherein when said fluid is electrified itprevents movement of said piston member and said insert member.
 20. Amethod for making an injection molded plastic article with at least onehollow portion, said method comprising the steps of: injecting a firstquantity of plastic material into a mold cavity; injecting a gasmaterial into said plastic material in said mold cavity; injecting asecond quantity of plastic material into said mold cavity to pack saidplastic material against the walls of said mold cavity; increasing thevolume of said mold cavity by displacing at least one insert memberpositioned in said mold cavity; continuing to inject the gas material inorder to maintain appropriate gas pressure in the plastic material;displacing a portion of said first or second plastic material into thevolume formerly occupied by said at least one insert member; displacingat least a portion of said injected gas into the portion of plasticmaterial formerly occupied by said portion of plastic materials whichwas displaced into said volume formerly occupied by said at least oneinsert member, thereby forming a hollow portion in a prespecifiedlocation in said plastic material; allowing the plastic material tosolidify in said mold cavity; and removing said plastic material nowforming a molded plastic article from said mold cavity.
 21. A method formaking an injection molded plastic article with at least one hollowportion, said method comprising the steps of: injecting a quantity ofplastic material into a mold cavity, the quantity of plastic materialbeing sufficient to completely fill the mold cavity; injecting aquantity of gas material into the plastic material in the mold cavity;displacing an insert member positioned in the mold cavity to form a voidin the mold cavity, while continuing to inject the gas material into theplastic material; filling said void with plastic material and forming ahollow portion in said plastic material; allowing said plastic materialto cool and solidify.
 22. The method as described in claim 21 furthercomprising the step of packing the quantity of plastic material in themold cavity prior to the gas injection.
 23. The method as described inclaim 21 further comprising the step of removing said cooled andsolidified plastic article from said mold.